Well ranging apparatus, systems, and methods

ABSTRACT

Disclosed embodiments include well ranging apparatus, systems, and methods which operate to measure electromagnetic field strength components associated with an electromagnetic field originating at a first well, via direct transmission or backscatter transmission, using at least one ranging electromagnetic field strength sensor attached to a housing, to provide ranging measurements. Further activities may include obtaining distorting field strength measurements using one or more reference electromagnetic field strength sensors, which may form or be attached in a closed loop path around the housing; and determining an approximate range between the first well and a second well in which the ranging electromagnetic field strength sensors are disposed, based on the ranging measurements and the distorting field strength measurements. Additional apparatus, systems, and methods are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of priority to provisionalapplication Ser. No. 62/035,877, filed Aug. 11, 2014; provisionalapplication Ser. No. 62/037,440, filed Aug. 14, 2014; and provisionalapplication Ser. No. 62/078,732, filed Nov. 12, 2014; each of which isincorporated herein by reference in its entirely.

BACKGROUND

With much of the world's easily obtainable oil having already beenproduced, new techniques are being developed to extract less accessiblehydrocarbons. These techniques often involve drilling a borehole inclose proximity to one or more existing wells. Examples of directeddrilling near an existing well include well intersection for blowoutcontrol, multiple wells drilled from an offshore platform, and closelyspaced wells for geothermal energy recovery. Another such technique issteam-assisted gravity drainage (SAGD) that uses a pair ofvertically-spaced, horizontal wells constructed along a substantiallyparallel path, often less than ten meters apart. Careful control of thespacing contributes to the effectiveness of the SAGD technique.

One way to construct a borehole in close proximity to an existing wellis “active ranging” or “access-dependent ranging” in which anelectromagnetic source is located in the existing well and monitored viasensors on the drill string in the well under construction. Anothertechnique involves systems that locate both the source and the sensor(s)on the drill string—relying on backscatter transmission from the targetwell to determine the range between the drilling well and the targetwell. These latter systems are sometimes called “passive ranging” or“access-independent” systems by those of ordinary skill in the art. Ineither case, the ranging techniques are sometimes limited in the degreeof accuracy that can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an example drilling environment in which rangingembodiments may be employed.

FIG. 2 illustrates perspective and top views of a well rangingapparatus, according to various embodiments.

FIGS. 3 to 9 illustrate a variety of apparatus, method, and systemconfigurations for various range determination embodiments.

FIG. 10 depicts an example drilling environment in which rangingembodiments may be employed.

FIGS. 11 to 13 are flow diagrams of well ranging methods, according tovarious embodiments.

FIG. 14 is a block diagram of a wireline system implementation ofvarious embodiments.

FIG. 15 is a block diagram of a drilling system implementation ofvarious embodiments.

DETAILED DESCRIPTION Introduction

Magnetic ranging has been widely used for various applications,including well intersection, well avoidance, SAGD, and others. Oneexcitation method for magnetic ranging is surface excitation. Surfaceexcitation is a popular method of generating a ranging signal. It isrelatively easy to implement, without the need for complex cabling andequipment. When surface excitation is used, a current is injected into atarget well casing at the surface of the well (e.g., at the well head).The current travels along the casing down-hole and generates a magneticfield down-hole that originates from the target via direct transmission,and can be measured at a distance (e.g., in a drilling well) for rangingpurposes. As a result, the excitation signal down-hole may be relativelyweak when the distance beneath the surface is great, due to the currentleakage into the conductive formation. Consequently, sensor noise oftenaffects magnetic ranging accuracy at greater depths, leading to falsesignal measurements and failures in well location. Some of theembodiments described herein are designed to improve down-hole currentstrength and/or enhance the signal/noise ratio, for improved accuracywith respect to ranging measurement technology.

Such apparatus, methods, and systems can be even more useful whenbackscatter ranging is used: that is, when the excitation source isinjected into the casing of the drilling well, or is attached to a drillstring within the drilling well. In the case of backscatter ranging, theexcitation source originates a direct transmission signal that impingesupon, and is then reflected from, the target well. When thesebackscatter transmission signals are received at a receiver in thedrilling well, the resulting received ranging signals are even weakerthan in the direct transmission case.

Thus, novel apparatus, methods, and systems are proposed to increase thestrength of the received ranging signal, to improve the receivedsignal-to-noise ratio (SNR), and to improve the accuracy of rangingsignal measurements. In some embodiments, enhancements are realized inall three of these areas. By taking this approach, ranging systemtechnology can be improved in a number of ways, via improved accuracyand reliability of individual ranging measurements. Therefore, theapparatus, methods, and systems proposed herein can be used to reducemeasurement issues that arise due to noise, as well as to generatelarger signals at great depths. The result is that the maximum detectionranges for existing ranging systems can be significantly improved. Insome embodiments, the apparatus, methods, and systems described hereincan be applied to electromagnetic (EM) telemetry applications.

FIG. 1 depicts an example drilling environment 100 in which rangingembodiments may be employed. The disclosed apparatus (e.g., loggingtools), systems, and methods are best understood in the context of thelarger systems in which they operate. Accordingly, FIG. 1 illustrates anexample drilling environment 100 in which a drilling platform 102supports a derrick 104 having a traveling block 106 for raising andlowering a drill string 108. A top drive 110 supports and rotates thedrill string 108 as it is lowered through the well-head 112. A drill bit114 is driven by a downhole motor and/or rotation of the drill string108. As the drill bit 114 rotates, it creates a borehole 116 that passesthrough various formations F. A pump 118 circulates drilling fluidthrough a feed pipe 120 to top drive 110, downhole through the interiorof drill string 108, through orifices in drill bit 114, back to thesurface via the annulus around drill string 108, and into a retentionpit 122. The drilling fluid transports cuttings from the borehole intothe retention pit 122 and aids in maintaining the borehole integrity.

The drill bit 114 is just one piece of a bottom-hole assembly thatincludes one or more drill collars (comprising thick-walled steel pipe)to provide weight and rigidity to aid the drilling process. Some ofthese drill collars include logging instruments to gather measurementsof various drilling parameters such as position, orientation,weight-on-bit, borehole diameter, etc. The tool orientation may bespecified in terms of a tool face angle (also known as rotational orazimuthal orientation), an inclination angle (the slope), and a compassdirection, each of which can be derived from measurements made bymagnetometers, inclinometers, and/or accelerometers, though other sensortypes such as gyroscopes may also be used. In one specific embodiment,the tool includes a three-axis fluxgate magnetometer and a three-axisaccelerometer. As is known in the art, the combination of these twosensor systems enables the measurement of the tool face angle,inclination angle, and compass direction. In some embodiments, the toolface and hole inclination angles are calculated from the accelerometersensor output, and the magnetometer sensor outputs are used to calculatethe compass direction.

The bottom-hole assembly further includes a ranging tool 124 to receivesignals from current injected by a surface power supply 148 into nearbyconductors such as pipes, casing strings, and conductive formations andto collect measurements of the resulting field to determine distance anddirection. Using measurements of these signals, in combination with thetool orientation measurements, the driller can, for example, steer thedrill bit 114 along a desired path in the drilling well 126 relative tothe existing well (e.g., target well) 128 in formation F using any oneof various suitable directional drilling systems, including steeringvanes, a “bent sub”, and a rotary steerable system. For precisionsteering, the steering vanes may be the most useful steering mechanism.The steering mechanism can be controlled from the Earth's surface, ordownhole, with a downhole controller programmed to follow the existingborehole 128 at a predetermined distance 130 and position (e.g.,directly above or below the existing borehole).

The ranging tool 124 may comprise one or more elements, interchangeablydesignated as receivers or sensors in this document. These elements maycomprise uniaxial, biaxial, or triaxial magnetometers, coil antennas,and/or telemetry receivers.

A telemetry sub 132 coupled to the downhole tools (including rangingtool 124) transmits telemetry data to the surface via mud pulsetelemetry. A transmitter in the telemetry sub 132 modulates a resistanceto drilling fluid flow to generate pressure pulses that propagate alongthe fluid stream at the speed of sound to the surface. One or morepressure transducers 134 convert the pressure signal into electricalsignal(s) for a signal digitizer 136. Note that other forms of telemetryexist and may be used to communicate signals from downhole to thedigitizer. Such telemetry may include acoustic telemetry,electromagnetic telemetry, or telemetry via wired drill pipe.

The digitizer 136 supplies a digital form of the telemetry signals via acommunications link 138 to a computer 140 or some other form of a dataprocessing device. The computer 140 operates in accordance with software(which may be stored on non-transitory information storage media 142)and user input provided via an input device 144 to process and decodethe received signals. The resulting telemetry data may be furtheranalyzed and processed by the computer 140 to generate a display ofuseful information on a computer monitor 146 or some other form of adisplay device. For example, a driller could employ this system toobtain and monitor drilling parameters, formation properties, and thepath of the borehole relative to the existing borehole 128 and anydetected formation boundaries. A downlink channel can then be used totransmit steering commands from the surface to the bottom-hole assembly.In some embodiments, the computer 140 has analog circuitry installed oris programmed to include a ranging determination module RD, whichoperates on the signal data received down hole at the ranging tool 124to determine the distance and direction from the drilling well 126 tothe target well 128. The ranging determination module RD may exist inthe computer 140 or the tool 124, and may be used to implement any ofthe methods described herein.

Thus, FIG. 1 illustrates an electromagnetic ranging system with surfaceexcitation. The power supply 148 at the surface employs a cable 150 toinject current into target well casing 152 and flowing down-hole so thatmagnetic fields can be generated surrounding a target well 128. Thensensors in the ranging tool 124 in the drilling well 126 can determinethe magnetic field strength in various directions so that distance anddirection between the target well 128 and drilling well 126 can bedetermined.

The drilling well 126 and the target well 128 are often constructed as acased hole, with cement installed around the outside of the casingmaterial (e.g., conductive piping). In the completion phase of oil andgas wells, the cement serves to isolate the wellbore, helps preventcasing failure, and keeps the wellbore fluids from contaminatingfreshwater aquifers.

For a two-sensor range measurement configuration, such as when bothsensors are mounted to a drill string in the same azimuthal plane, aninsert current may result from bottom hole assembly (BHA) conductivecurrent. In this case, the insert current flows from the drill collar tothe insert, and exists inside the measurement radius of the sensors. Anycurrent that flows within the sensor radius can distort the rangingmeasurement. These distorting currents can also flow through othercomponents within the well, such as drilling mud.

As a solution to this technical problem, the inventors have developed avariety of apparatus, systems, and methods to identify insert currenteffects on ranging sensor measurements, and to reduce these effects. Theresult of implementing various embodiments may be improved accuracy whendetermining the range between wells. Sensor noise effects on rangingperformance may also be reduced. Several embodiments that may providesome of these advantages will now be described.

Detailed Presentation

In a first set of embodiments, the effect of the distorting current maybe offset by installing multiple reference sensors in a circular path,approximately matching the radius of the ranging measurement sensors.The reference sensors may be installed along other radii, using a largeror smaller radius, but it may be easier to average measurements alongthe circle of reference sensors when all sensors (e.g., the regularmeasurement sensors and reference sensors) are located at the sameradial distance from the tool longitudinal centerline. The greater thenumber of reference sensors that are used, the more the measurementaccuracy results should improve.

In a second set of embodiments, a closed path reference sensor is usedto determine the magnitude of the insert current. The insert currentmagnitude is then used to calibrate measurements made by one or moreranging sensors.

FIG. 2 illustrates perspective and top views of a well ranging apparatus200, according to various embodiments. Here ranging sensors S1, S2 areshown. According to the first set of embodiments, their measurementaccuracy can be improved by the addition of reference sensors RS3, RS4,RS5, and RS6. As shown, all sensors (S1, S2, RS3, RS4, RS5, and RS6) areattached to a down hole housing, such as a ranging tool or BHA, atapproximately the same radial distance from the housing centerline 210.

In surface excitation applications, excitation current may be injectedinto a target well, with sensors located in a drilling well, perhaps inthe BHA. The sensors are utilized to detect the signals generated by thetarget well current, and thereafter determine the relative rangingdistance and direction between the target well and the drilling well.However, it has been determined that a portion of the surface excitationcurrent will flow from the target well into the drilling well, causingso-called leakage current in the BHA. The flow of leakage current, inturn, will introduce current flowing in the tool insert, owing toconductive materials in the insert. The insert current will thenintroduce an additional signal into the sensors, disturbing the sensormeasurements, and affecting ranging performance accuracy. Consequently,this disclosure describes detailed processing methods and correspondingtool configurations to determine the magnitude of the insert current,and to reduce the effect of the insert current on sensor measurementsfor ranging applications.

Fundamentals of Range Determination

FIGS. 3 to 9 illustrate a variety of apparatus, method, and systemconfigurations for various range determination embodiments. To begin,the reader is referred to FIG. 3, which shows the magnetic field H foran infinite line source 300 with a constant current I. Based on Ampere'slaw, the magnetic field H at low frequency surrounding the line source300 can be expressed as:

$\begin{matrix}{{\overset{\rightharpoonup}{H} = {\frac{I}{2\pi \; r}\hat{\Phi}}},} & (1)\end{matrix}$

where r is the distance between the measurement point and the infiniteline source. In addition, the gradient field can be given by:

$\begin{matrix}{\frac{\partial\overset{\rightharpoonup}{H}}{\partial r} = {{- \frac{I}{2\pi \; r^{2}}}\hat{\Phi}}} & (2)\end{matrix}$

Consequently, the distance r can be directly computed by taking a ratioof the amplitude of Equation (1) to amplitude of Equation (2), given by:

$\begin{matrix}{{\frac{\overset{\rightharpoonup}{H}}{\frac{\partial\overset{\rightharpoonup}{H}}{\partial r}}} = {{\frac{\frac{I}{2\pi \; r}}{\frac{- I}{2\pi \; r^{2}}}} = {r.}}} & (3)\end{matrix}$

Equation (3) may be designated as the gradient method used to computeranging distance. In practice, two sensors (e.g., magnetometers) can beused to make magnetic field and gradient field measurements. This isshown in FIG. 4, which illustrates an infinite line source and a loggingtool equipped with two sensors S1, S2 for gradient field determination.

A finite difference method is then utilized to calculate the magneticfield strength H and the gradient field strength, given by:

$\begin{matrix}{{\overset{\rightharpoonup}{H} = \frac{{\overset{\rightharpoonup}{H}}_{1} + {\overset{\rightharpoonup}{H}}_{2}}{2}},{and}} & \left( {4a} \right) \\{{\frac{\partial\overset{\rightharpoonup}{H}}{\partial r} = \frac{{\overset{\rightharpoonup}{H}}_{1} + {\overset{\rightharpoonup}{H}}_{2}}{2\Delta \; S}},} & \left( {4b} \right)\end{matrix}$

where H₁ and H₂ are the total field measurements at sensors S1 and S2,respectively. ΔS is the separation between a sensor and the tool center.Consequently, Equation (3) can be modified based on the finitedifference method to compute the ranging distance r, as:

$\begin{matrix}{r = {{\frac{\frac{{\overset{\rightharpoonup}{H}}_{1} + {\overset{\rightharpoonup}{H}}_{2}}{2}}{\frac{{\overset{\rightharpoonup}{H}}_{1} + {\overset{\rightharpoonup}{H}}_{2}}{2\Delta \; S}}}.}} & (5)\end{matrix}$

In practice, each sensor measures three orthogonal field components toacquire the total field measurement. The three orthogonal fieldcomponents are the normal component, the tangential component, and the zcomponent, as shown in FIG. 5. As shown in the figure, the normalcomponent and tangential component are in the same plane as theazimuthal plane of the tool 510, which may be a logging tool. Thedirection of the tangential component is the tool rotation direction,whereas the normal direction is perpendicular to the tool rotationdirection and points away from the tool center; it lies on a straightline between the tool center and the sensor location. The z componentdirection is parallel to the tool mandrel (i.e., the longitudinal axisof the tool) along with the BHA.

With these definitions, it can be seen that the total field (H₁ or H₂ inFIG. 4) can be determined by Equation (6), as:

Total Field {right arrow over (H)}={right arrow over (H)} _(Z) +{rightarrow over (H)} _(Tangential) +{right arrow over (H)} _(Normal).  (6)

The amplitude of the total field can be calculated by Equation (7), as:

|{right arrow over (H)}|√{square root over (|{right arrow over (H)}_(Z)|² +|{right arrow over (H)} _(Tangential)|² +|{right arrow over (H)}_(Normal)|²)}.  (7)

Leakage Current in the Tool Insert

FIG. 6 is a top view of two line sources, one provided by the targetcurrent, and the other by the leakage current, in the context of alogging tool 610 equipped with two sensors S1, S2 for gradient fielddetermination. The issue of leakage current flowing in the tool insertintroduces an additional current source flowing at the center of thetool 610 between the two sensors S1, S2. Assuming the additionalcurrent, that is the leakage current I_(Leak), is uniformly flowing atthe tool center, and that the additional tangential field componentsH_(1Leak) and H_(2Leak) will affect measurements made by the sensors S1and S2, respectively, it can be seen that the leakage current I_(Leak)can generate interfering signals—disturbing ranging calculations basedon total field or tangential component calculations that make use ofsensor measurements provided by sensors S1, S2.

On the other hand, the normal component of sensor measurements is notaffected by this leakage current, since the field orientation of thenormal component is different from the field orientation of the leakagecurrent, and therefore a ranging determination based only on the normalcomponent should have higher ranging accuracy than one using other fieldcomponents (e.g., tangential or total field).

Determining Insert Current and Reducing Insert Current MeasurementEffects

The insert current described above can be calculated based on Ampere'slaw, given by:

$\begin{matrix}{{\oint\limits_{C}{\overset{\rightharpoonup}{H}{\overset{\rightharpoonup}{r}}}} = {I_{S} + {\int_{S}{{\frac{\partial\overset{\rightharpoonup}{D}}{\partial t} \cdot \hat{n}}{{S}.}}}}} & (8)\end{matrix}$

As shown in FIG. 7, following the right-hand rule, the line integral ofthe magnetic H field around a closed curve Cd{right arrow over (r)} isan infinitesimal element along the curve C, and equals the current I_(S)through a surface S.

Since low frequency operation is used in many ranging applications,Equation (8) can often be simplified as:

$\begin{matrix}{{\oint\limits_{C}{\overset{\rightharpoonup}{H}{\overset{\rightharpoonup}{r}}}} = {I_{S}.}} & (9)\end{matrix}$

As shown in FIG. 8, which presents perspective and end views of a toolinsert 800, to determine the insert current, one can choose the lineintegral path 810 for the curve C (highlighted as a dashed line in thefigure) as the outer circle of the insert 800. Due to the symmetricalstructure of the tool insert 800, one can treat the total current I_(Z)flowing in the insert cross section surface S as a single line sourceflowing at the tool center. The insert current flows in the z directionand can be calculated by Equation (9). To carry out the calculation,various sensors located along the path of the curve C can be used tomeasure the tangential component at each sensor location.

For simplicity, N sensors can be installed and equally spaced atdifferent tool azimuth locations along the path of the curve C.Therefore, the tangential component of measurements made by each sensorS₁, S₂, S₃, . . . , S_(N-1), S_(N) in FIG. 8 can be utilized to estimatethe insert current, based on Equation (9), as:

$\begin{matrix}\begin{matrix}{I_{Z} = {\oint\limits_{C}{\overset{\rightharpoonup}{H}{\overset{\rightharpoonup}{r}}}}} \\{{\approx {\sum\limits_{i = 1}^{N}\; {{H_{tangenial}(i)} \times \left( {2\pi \; r\; \Delta \; S} \right) \times \frac{2\pi}{N}}}},}\end{matrix} & (10)\end{matrix}$

where H_(tangential)(i) is the tangential component measurement atsensor i in the figure, for i=1 to N.

Once the insert current I_(Z) (or I_(Leak) in FIG. 6) is determined byEquation (10), the field H₁ _(Leak) and H₂ _(Leak) in FIG. 6 can beexpressed as:

$\begin{matrix}{H_{1_{Leak}} = {H_{2_{Leak}} = {\frac{I_{Z}}{2\pi \; r\; \Delta \; S}.}}} & (11)\end{matrix}$

Thus, the field strength {right arrow over (H)}_(Target) (i.e., H₁ or H₂in FIG. 6) due to target well current (I in FIG. 4) can be determined by

{right arrow over (H)} _(Target) ={right arrow over (H)} _(Total)−{right arrow over (H)} _(Leakage),  (12)

where {right arrow over (H)}_(Total) indicates total field measurement(including tangential, normal and z components) at each sensor, and{right arrow over (H)}_(Leakage) is the field strength due to insertcurrent (H₁ _(Leak) and H₂ _(Leak) in FIG. 6) that is calculated byEquation (11). Here wherein {right arrow over (H)}_(Target) comprises acorrected field measurement that can be used to determine the rangebetween two wells, as follows.

Equation (12) denotes a directional vector for each field, whereas thetotal field direction can be calculated based on all three componentmeasurements (tangential, normal, and z) and the leakage field strengthis in the tangential directional of each sensor location. Therefore, thefield direction for {right arrow over (H)}_(Target) can be determinedbased on Equation (12), after which the ranging distance to the targetwell using the measurements provided by two sensors ({right arrow over(H)}_(Target) at sensors S1, S2 in FIG. 6) can be made, benefiting froma reduced insert current effect on the ranging calculations.

It is noted that installing a greater number of reference sensors(measuring the tangential component) in the tool insert should increasethe accuracy that will be realized with respect to insert currentcalculation using Equation (11). That is, the integral approximationwill be more accurate when more sensors are used. At least two sensorsS1, S2 should be used for calculations with Equation (11). However,instead of installing multiple sensors to provide the input measurementdata for Equation (11), a field operator can rotate the tool 800 withone sensor S1 to a variety of tool azimuth locations, perhaps separatedby a distance of 2π/N radians, where N is the number of locations, andthen use the sensor S1 to take measurements at each of the differentazimuthal angles around the insert outer circle path 810. Theseazimuthal measurements can thereafter be used to provide the data foruse with Equation (11), without installing multiple sensors in the tool.

It is also noted that the sensors (S1, . . . SN) shown in FIG. 8, withmany embodiments using at least three sensors, do not necessarily haveto be equally spaced in the azimuthal direction. If the sensors are notequally spaced around the azimuthal direction, Equation (10) can bemodified as shown here:

$\begin{matrix}\begin{matrix}{I_{Z} = {\oint\limits_{C}{\overset{\rightharpoonup}{H}{\overset{\rightharpoonup}{r}}}}} \\{\approx {\sum_{i = 1}^{N}\; {{H_{tangenial}(i)} \times \left( {2\pi \; r\; \Delta \; S} \right) \times {\beta_{i}.}}}}\end{matrix} & (13)\end{matrix}$

In this case, β_(i) can be expressed as:

$\begin{matrix}{\beta_{i} = {{{\alpha_{i} - \alpha_{i - 1}}}\mspace{14mu} {and}}} & \left( {14a} \right) \\{{\beta_{i} = {{\alpha_{i + 1} - \alpha_{i}}}},{or}} & \left( {14b} \right) \\{{\beta_{i} = \frac{{{{\alpha_{i} - \alpha_{i - 1}}} - {{\alpha_{i + 1} - \alpha_{i}}}}\mspace{11mu}}{2}},} & \left( {14c} \right)\end{matrix}$

where α_(i) is the tool azimuth angle for sensor i, as i varies from 1to N. The range adjustment module RA shown in FIG. 2 can be used to makethe adjustments to the ranging sensor measurements, according to thereference sensor data.

For an explanation of the second set of embodiments mentionedpreviously, it is noted that a reference electromagnetic field strengthsensor can be used to determine the magnitude of the insert currentI_(z) in Equation (13). Thus, as shown in FIG. 9, a referenceelectromagnetic field strength sensor 910, such as a toroidal antenna,can be mounted on a down hole housing 900 (e.g., on a wireline sonde1410 in FIG. 14, or on a logging tool forming part of the bottom holeassembly 1526 in FIG. 15 of a drill string) proximate to the rangingelectromagnetic field strength sensors S1 . . . SN, as shown in FIG. 9,or underneath the ranging electromagnetic field strength sensors S1 . .. SN, at the location of the dashed line 920. The referenceelectromagnetic field strength sensor 910 can be used to directlydetermine the magnitude of the magnetic B field (B_(insert)) due to theinsert current.

It should be noted that the reference electromagnetic field strengthsensor 910 can be any one of a number of shapes, including circular,square, multi-sided (e.g., triangular, hexagonal, etc.)—as long as thereference electromagnetic field strength sensor 910 provides a closedloop current path around the tool housing 900.

For example, if a toroidal antenna is used as the referenceelectromagnetic field strength sensor 910, the magnitude of thedistorting field strength B_(insert) due to the insert current I_(z)(i.e., the distorting field strength measurement) can be obtained bymeasuring the voltage in the toroid, combined with the knowledge of thenumber of turns (M) around the circumference of the toroid. Thus, theinsert current I_(Z) is proportional to the voltage received by thetoroid V_(Toroid)=jωB_(insert)MA according to Equations (15) and (16),as follows:

$\begin{matrix}{{B_{insert} = {V_{Toroid}/\left( {{j\omega}\; {MA}} \right)}},{and}} & (15) \\\begin{matrix}{I_{Z} = {\oint_{C}{\overset{\rightharpoonup}{H}{\overset{\rightharpoonup}{r}}}}} \\{= {\frac{B_{insert}}{\mu} \times 2\pi \; L}} \\{= \frac{V_{Toroid}\left( {2\pi \; L} \right)}{{j\omega\mu}\; {MA}}}\end{matrix} & (16)\end{matrix}$

where L is the radius of the toroid, μ is the magnetic permeability ofthe toroid, and A is the area of each of the M-loops of the toroid.After the field strength B_(insert) due to the insert current has beenmeasured, the measurements provided by the ranging electromagnetic fieldstrength sensors can be calibrated according to Equations (15) and (16).

FIG. 10 depicts an example drilling environment 1000 in which rangingembodiments may be employed. In some embodiments, an apparatus comprisesa down hole housing (e.g., ranging tool 124) attached to a rangingelectromagnetic field strength sensor, such as one or more of thesensors S1, S2. The ranging electromagnetic field strength sensor can beoperated to measure electromagnetic field strength components associatedwith an electromagnetic field originated by a current source (e.g, thepower supply P.S.), via direct transmission or backscatter transmission,when the housing is disposed in the drilling well 126, to provideranging measurements for calculating an approximate range 130 betweenthe target well 128 and the drilling well 126, and a referenceelectromagnetic field strength sensor (e.g., as part of apparatus 200)attached in a closed loop path around the housing, to provide distortingfield strength measurements. A system may include the apparatus shown inFIG. 10, as well as a current source to couple current to the targetwell or a the drilling well.

In some embodiments, a down hole housing (e.g., the ranging tool 124) isattached to a ranging electromagnetic field strength sensor, such as oneor more of the sensors S1, S2, the electromagnetic field strengthsensor(s) to measure electromagnetic field strength componentsassociated with an electromagnetic field originating at a first well(e.g., a target well 128), via direct transmission or backscattertransmission, when the housing is disposed in a second well (e.g., adrilling well 126), to provide ranging measurements for calculating anapproximate range between the first well and the second well. Theapparatus may further comprise a reference electromagnetic fieldstrength sensor (e.g., perhaps comprising one or more sensors RS3 . . .RS6 of FIG. 2) providing a closed loop current path around the housing,to provide distorting field strength measurements.

When it is known ahead of time that a particular well will be used as aranging target (e.g., the target well 128), a device DV can bepermanently installed along with or in that well as part of the wellcompletion process, before production begins. The device DV may useelectromagnetic waves in a variety of ways. For example, the device DVmay comprise a conductor or an insulated conductor, such as a piece ofwire or cable, or a coaxial cable, embedded in the well casing. Thedevice DV may include a solenoid or switch connected to a source ofpower, to receive an electrical signal, to apply the power to theconductor, and by that action, to produce a magnetic field originatingfrom the ranging target well that can be measured by sensors (e.g.,magnetometers) in a drilling well. In some embodiments, the device DVmay comprise a waveguide to receive energy through the target wellcasing. The device DV may comprise a capacitor or inductor to capture anelectrical field (voltage difference) that can in turn be measuredremotely, at the drilling well.

In some embodiments an apparatus comprises a range determination moduleRD (see FIGS. 1, 10, and 14-15) to receive the ranging measurements fromthe ranging electromagnetic field strength sensors S1 . . . SN anddistorting field strength measurements from a reference electromagneticfield strength sensor, such as a toroidal antenna, shown in FIG. 9. Therange determination module RD may comprise a processing unit programmedto implement any of the calculations shown as part of the equations inthis document, to provide adjusted values of the ranging measurements tocalculate the approximate range to the target well.

It should be noted that when the insert current sensor (i.e., thereference electromagnetic field strength sensor) is used to receive thedistributed magnetic field, a single ranging electromagnetic fieldstrength sensor can be employed to determine the range to the targetwell. That is, while a single ranging sensor is normally unable toprovide the gradient field for ranging distance calculations, thecombination of the single ranging sensor and the insert current sensorenable dual measurements of the ranging signal provided by the targetwell—so that in some embodiments a single ranging sensor, plus theinsert current sensor, are sufficient to provide the desired rangingdistance determination.

FIGS. 11 to 13 are flow diagrams of well ranging methods 1111, 1211,1311 according to various embodiments. Referring now to FIG. 11, amethod of range determination between wells, using ranging sensormeasurements adjusted according to reference sensor measurements, can beseen.

At block 1121, the method 1111 comprises measuring electromagnetic fieldstrength components associated with an electromagnetic field originatingat a first well, via direct transmission or backscatter transmission,using at least two ranging electromagnetic field strength sensorsattached to a housing, to provide ranging measurements. The total Hfield can be measured for each sensor.

In some embodiments, the measurements of the H field made at block 1121are designated as distorting field strength measurements. That is, theactivity at block 1121 may further comprise obtaining distorting fieldstrength measurements using a set of at least three referenceelectromagnetic field strength sensors attached in a circular patharound the housing.

In some embodiments, the H field measurements obtained at block 1121 areaveraged, at block 1125.

In blocks 1133, 1137, and 1141, the method 1111 may comprise determiningthe approximate range between the first well and a second well in whichthe ranging electromagnetic field strength sensors are disposed, basedon the ranging measurements and the distorting field strengthmeasurements. This may occur by way of: calculating the distorting fieldfor each of the sensors at block 1133, subtracting the original sensor Hfield measurement (made at block 1121) form the calculated distortingfield for each sensor at block 1137, and then using the corrected Hfield measurements to determine the range between the wells at block1141. The approximate range may be determined according to the formula{right arrow over (H)}_(Target)={right arrow over (H)}_(Total)−{rightarrow over (H)}_(Leakage). In this case, the set of referenceelectromagnetic field strength sensors may be attached in a circularpath around the housing, to include the two ranging electromagneticfield strength sensors.

In FIG. 12, another ranging method 1211 embodiment is illustrated. Herethe method 1211 begins at block 1221 with first measuring a totalelectromagnetic field value for a ranging sensor disposed in a secondwell, wherein a signal to be measured originates at a first well, andthe first measuring occurs in the second well.

In some embodiments, the method 1211 continues on to block 1225 toinclude second measuring a total electromagnetic field value of areference sensor attached in a closed loop around a housing wherein theranging sensor and the reference sensor are located on the same housing.

In some embodiments, the method 1211 continues on to block 1229 toinclude calculating a distorting current value based on the totalelectromagnetic field value of the reference sensor.

In some embodiments, the method 1211 continues on to block 1233 toinclude determining a distorting field value for the ranging sensorbased on the calculated distorting current value of the reference device

In some embodiments, the method 1211 continues on to block 1237 toinclude determining a difference between the total electromagnetic fieldvalue measured by the ranging sensor and the distorting field valuedetermined by the reference device, to provide a correctedelectromagnetic field value measurement.

In some embodiments, the method 1211 continues on to block 1243 toinclude determining an approximate range between the first well and thesecond well using the corrected electromagnetic field value measurement.

In FIG. 13, another ranging method 1311 embodiment is illustrated. Herethe method 1311 begins at block 1321 with measuring electromagneticfield strength components associated with an electromagnetic fieldoriginating at a first well, via direct transmission or backscattertransmission, using at least one ranging electromagnetic field strengthsensor attached to a housing, to provide ranging measurements

In some embodiments, the method 1311 continues on to block 1325 toinclude obtaining distorting field strength measurements using areference electromagnetic field strength sensor attached in a closedloop path around the housing.

In some embodiments, the method 1311 continues on to block 1229 toinclude determining an approximate range between the first well and asecond well in which the ranging electromagnetic field strength sensorsare disposed, based on the ranging measurements and the distorting fieldstrength measurements.

Additional Detailed Description and Some Representative Embodiments

FIG. 14 is a block diagram of a wireline system 1400 implementation ofvarious embodiments. The system 1400 of FIG. 14 may include any of theembodiments of receiver or sensor mounting discussed previously. In thiscase, a hoist 1406 may be included as a portion of a platform 1402, suchas might be coupled to a derrick 1404, and used to raise or lowerequipment such as a wireline sonde 1410 into or out of a borehole. Thewireline sonde 1410 may include any one or more of the above-describedembodiments, including sensors (e.g., shown as apparatus 200 and 910)and a range determination module RD, and/or range adjustment module RA.

In this wireline example, a cable 1442 may provide a communicativecoupling between a logging facility 1444 (e.g., including a processorcircuit 1445 including memory or other storage or control circuitry) andthe sonde 1410. In this manner, information about the formation 1418 maybe obtained. The processor circuit 1445 can be configured to access andexecute instructions stored in a memory to implement any of the methodsdescribed herein (e.g., by accessing a range determination module RD orrange adjustment module RA).

FIG. 15 is a block diagram of a drilling system 1500 implementation ofvarious embodiments. This diagram shows a drilling rig system 1500according to various embodiments that may include measurement whiledrilling (MWD) or logging while drilling (LWD) capability. The drillingapparatus can use data from an insert in the drill string 1508, havingattached to a number of receivers or sensors (e.g., shown as apparatus200 and 910) as discussed previously, and using acquired and calculatedranging information to steer the drill bit 1514.

A drilling rig or platform 1402 generally includes a derrick 1404 orother supporting structure, such as including or coupled to a hoist1406. The hoist 1406 may be used for raising or lowering equipment orother apparatus such as drill string 1508. The drill string 1508 mayaccess a borehole 1516, such as through a well head 1412. The lower endof the drill string 1508 may include various apparatus, such as a drillbit 1514, such as to provide the borehole 1516.

A drilling fluid or “mud” may be circulated in the annular region aroundthe drill bit 1514 or elsewhere, such as provided to the borehole 1516through a supply pipe 1522, circulated by a pump 1520, and returning tothe surface to be captured in a retention pit 1524 or sump. Various subsor tool assemblies may be located along the drill string 1508, such as abottom hole assembly (BHA) 1526 or a second sub 1528. The BHA 1526and/or the sub 1528 may include one or more sensors or receivers (e.g.,shown as apparatus 200 and 910), as described herein, along with acurrent source to initiate a ranging signal, and a processor with accessto a memory that contains a program to implement any of the methodsdescribed herein (e.g., a ranging determination module RD, and/or aranging adjustment module RA).

Thus, some of the embodiments described herein may be realized in part,as a set of instructions on a computer readable medium 142 comprisingROM, RAM, CD, DVD, hard drive, flash memory device, or any othercomputer readable medium, now known or unknown, that when executedcauses a computing system, such as computer as illustrated in FIG. 1 or10, or some other form of a data processing device 140, to implementportions of a method of the present disclosure, for example theprocesses and methods described in FIGS. 11-13 (e.g., forcomputer-assisted well completion).

Though sometimes described serially in the examples of FIG. 11-13, oneof ordinary skill in the art would recognize that other examples mayreorder the operations, omit one or more operations, and/or execute twoor more operations in parallel using multiple processors or a singleprocessor organized as two or more virtual machines or sub-processors.Moreover, still other examples can implement the operations as one ormore specific interconnected hardware or integrated circuit modules withrelated control and data signals communicated between and through themodules. Thus, any process flow is applicable to software, firmware,hardware, and hybrid implementations.

It is expected that the system range and performance can be extendedwith the various embodiments described herein. Power can often be saved,and accuracy of ranging measurements improved. Signal components may beextracted and converted to pixel colors or intensities and displayed asa function of tool position and azimuth. Assuming the target casingstring is within detection range, it may appear as a bright (or, ifpreferred, a dark) band in the image. The color or brightness of theband may indicate the distance to the casing string, and the position ofthe band indicates the direction to the casing string. Thus, by viewingsuch an image, a driller can determine in a very intuitive mannerwhether the new borehole is drifting from the desired course and he orshe can quickly initiate corrective action. For example, if the bandbecomes dimmer, the driller can steer towards the casing string.Conversely, if the band increases in brightness, the driller can steeraway from the casing string. If the band deviates from its desiredposition directly above or below the casing string, the driller cansteer laterally to re-establish the desired directional relationshipbetween the boreholes.

While the text of this document has been divided into sections, itshould be understood that this has been done as a matter of convenience,and that the embodiments discussed in any one section may form a part ofany or more embodiments described in another section, and vice-versa.Moreover, various embodiments described herein may be combined with eachother, without limitation. Thus, many embodiments may be realized.

Similarly, while some of the above-described embodiments may show onlyone receiver, perhaps in the form of a magnetometer, coil, or telemetryreceiver, one of ordinary skill in the art would realize that a drillstring or downhole tool may include multiple receivers for making thevarious measurements described herein. Examples of various embodimentswill now be listed in a non-limiting fashion.

In some embodiments, a method comprises measuring electromagnetic fieldstrength components associated with an electromagnetic field originatingat a first well, via direct transmission or backscatter transmission,using two ranging electromagnetic field strength sensors attached to ahousing, to provide ranging measurements.

In some embodiments, the method comprises obtaining distorting fieldstrength measurements using a set of at least three referenceelectromagnetic field strength sensors attached in a circular patharound the housing.

In some embodiments, the method comprises determining the approximaterange between the first well and a second well in which the rangingelectromagnetic field strength sensors are disposed, based on theranging measurements and the distorting field strength measurements.

In some embodiments, the approximate range is determined according tothe formula {right arrow over (H)}_(Target)={right arrow over(H)}_(Total)−{right arrow over (H)}_(Leakage), where {right arrow over(H)}_(Total) comprises a total field measurement at each sensor, wherein{right arrow over (H)}_(Leakage) comprises a field strength due to aninsert current, and wherein {right arrow over (H)}_(Target) comprises acorrected field measurement.

In some embodiments, the set of reference electromagnetic field strengthsensors attached in the circular path around the housing includes thetwo ranging electromagnetic field strength sensors.

In some embodiments, a method comprises measuring a totalelectromagnetic field value for each one of a set of ranging sensorsdisposed in a second well, wherein a signal to be measured originates ata first well, and the measuring occurs in the second well.

In some embodiments, the method comprises averaging the totalelectromagnetic field value over all of the ranging sensors to providean average total field value.

In some embodiments, the method comprises calculating an averagedistorting current value based on the average total field value.

In some embodiments, the method comprises determining an averagedistorting field value based on the average distorting current value.

In some embodiments, the method comprises determining a differencebetween the total electromagnetic field value measured by each of theranging sensors and the average distorting field value, to providecorrected electromagnetic field value measurements.

In some embodiments, the method comprises determining an approximaterange between the first well and the second well using the correctedelectromagnetic field value measurements.

In some embodiments, a method comprises first measuring a totalelectromagnetic field value for a ranging sensor disposed in a secondwell, wherein a signal to be measured originates at a first well, andthe first measuring occurs in the second well.

In some embodiments, the method comprises second measuring a totalelectromagnetic field value of a reference sensor attached in a closedloop around a housing wherein the ranging sensor and the referencesensor are located on the same housing.

In some embodiments, the method comprises calculating a distortingcurrent value as a calculated distorting current value, based on thetotal electromagnetic field value of the reference sensor.

In some embodiments, the method comprises determining a distorting fieldvalue for the ranging sensor based on the calculated distorting currentvalue.

In some embodiments, the method comprises determining a differencebetween the total electromagnetic field value measured by the rangingsensor and the distorting field value, to provide a correctedelectromagnetic field value measurement.

In some embodiments, the method comprises determining an approximaterange between the first well and the second well using the correctedelectromagnetic field value measurement.

In some embodiments, a method comprises measuring electromagnetic fieldstrength components associated with an electromagnetic field originatingat a first well, via direct transmission or backscatter transmission,using at least one ranging electromagnetic field strength sensorattached to a housing, to provide ranging measurements.

In some embodiments, the method comprises obtaining distorting fieldstrength measurements using a reference electromagnetic field strengthsensor attached in a closed loop path around the housing.

In some embodiments, the method comprises determining an approximaterange between the first well and a second well in which the rangingelectromagnetic field strength sensors are disposed, based on theranging measurements and the distorting field strength measurements.

In some embodiments, as shown in FIGS. 1-10 and 14-15, an apparatuscomprises a down hole tool housing attached to at least two rangingelectromagnetic field strength sensors, each of the electromagneticfield strength sensors to measure electromagnetic field strengthcomponents associated with an electromagnetic field originating at afirst well, via direct transmission or backscatter transmission, whenthe housing is disposed in a second well, to provide rangingmeasurements for calculating an approximate range between the first welland the second well. In some embodiments, the apparatus comprises a setof at least three reference electromagnetic field strength sensorsattached in a circular path around the housing, to provide distortingfield strength measurements.

In some embodiments, the ranging electromagnetic field strength sensorsand the reference electromagnetic field strength sensors are spacedapart from each other in an azimuthal direction, and located atapproximately a same radial distance from a longitudinal centerline ofthe housing. In some embodiments, the ranging electromagnetic fieldstrength sensors are spaced substantially equally apart from each otherin an azimuthal direction.

In some embodiments, the reference electromagnetic field strengthsensors are spaced substantially equally apart from each other along thecircular path. In some embodiments, the set of reference electromagneticfield strength sensors includes the two ranging electromagnetic fieldstrength sensors.

In some embodiments, an apparatus comprises a down hole tool housingattached to a ranging electromagnetic field strength sensor, the rangingelectromagnetic field strength sensor to measure electromagnetic fieldstrength components associated with an electromagnetic field originatingat a first well, via direct transmission or backscatter transmission,when the housing is disposed in a second well, to provide rangingmeasurements for calculating an approximate range between the first welland the second well.

In some embodiments, the apparatus comprises a reference electromagneticfield strength sensor providing a closed loop current path around thehousing, to provide distorting field strength measurements.

In some embodiments, the reference electromagnetic field strength sensorcomprises a toroidal antenna. In some embodiments, the referenceelectromagnetic field strength sensor is attached to the rangingelectromagnetic field strength sensor, or to the housing.

In some embodiments, as shown in FIGS. 1-10 and 14-15, a systemcomprises a current source to couple current to a target well or adrilling well. The system may further comprise an apparatus, theapparatus comprising a down hole tool housing attached to two rangingelectromagnetic field strength sensors. Each of the electromagneticfield strength sensors may operate to measure electromagnetic fieldstrength components associated with an electromagnetic field originatedby the current source, via direct transmission or backscattertransmission, when the housing is disposed in the drilling well, toprovide ranging measurements for calculating an approximate rangebetween the target well and the drilling well. The system may furthercomprise a set of at least three electromagnetic field strength sensorsattached in a circular path around the housing, to provide distortingfield strength measurements.

In some embodiments, the system comprises a range adjustment module toreceive the ranging measurements and the distorting field strengthmeasurements, and to provide adjusted values of the ranging measurementsto calculate the approximate range. In some embodiments, the set ofreference electromagnetic field strength sensors attached in thecircular path around the housing includes the two rangingelectromagnetic field strength sensors.

In some embodiments, a system comprises a current source to couplecurrent to a target well or a drilling well, as well as an apparatuscomprising a down hole tool housing attached to a rangingelectromagnetic field strength sensor, the ranging electromagnetic fieldstrength sensor to measure electromagnetic field strength componentsassociated with an electromagnetic field originated by the currentsource, via direct transmission or backscatter transmission, when thehousing is disposed in the drilling well, to provide rangingmeasurements for calculating an approximate range between the targetwell and the drilling well. The system may further comprise a referenceelectromagnetic field strength sensor attached in a closed loop patharound the housing, to provide distorting field strength measurements.

In some embodiments, the reference electromagnetic field strength sensoris used to measure the electromagnetic field strength componentsassociated with the electromagnetic field originating at the first well,via the direct transmission or the backscatter transmission, inconjunction with the ranging electromagnetic field strength sensor.

In some embodiments, the system comprises a range determination moduleto receive the ranging measurements and the distorting field strengthmeasurements, and to provide adjusted values of the ranging measurementsto calculate the approximate range.

In some embodiments, an apparatus comprises a down hole tool housing(e.g., ranging tool 124) attached to a set of sensors, the down holetool housing comprising one or more of a wireline sonde, a bottom holeassembly, a drill collar, a drill string pipe, or a sub. Someembodiments of this apparatus further comprise a processor (e.g.,computer 140) communicatively coupled to the set of sensors to receiveelectromagnetic signal strength signals from the sensors, and to amemory (e.g., medium 142), the memory having a set of instructionswhich, when executed by the processor, cause the processor to implementany of the methods described herein.

In some embodiments, a system comprises a source of current or voltage(e.g., power supply 148) to electrically couple to a well casing of afirst well or to attach to a first down hole tool housing. Someembodiments of this system further comprise a drill string to bedisposed in a second well and mechanically coupled to a second down holetool housing, the second down hole tool housing attached to a set ofsensors. Some embodiments of this system further comprise a processor(e.g., computer 140) communicatively coupled to the set of sensors toreceive signals representing electromagnetic field strength from thesensors, in response to the source exciting the well casing directly toinitiate direct signal transmission, or indirectly via backscattertransmission, the processor communicatively coupled to a memory (e.g.,medium 142) having a set of instructions which, when executed by theprocessor, cause the processor to implement any of the methods describedherein.

Numerous other variations and modifications will become apparent tothose skilled in the art once the above disclosure is fully appreciated.For example, the foregoing discussion has focused on a logging whiledrilling implementation, but the disclosed techniques would also besuitable for wireline tool implementation (as shown in FIG. 14). It isintended that the following claims be interpreted to embrace all suchvariations and modifications.

In this description, references to “one embodiment” or “an embodiment,”or to “one example” or “an example” mean that the feature being referredto is, or may be, included in at least one embodiment or example of theinvention. Separate references to “an embodiment” or “one embodiment” orto “one example” or “an example” in this description are not intended tonecessarily refer to the same embodiment or example; however, neitherare such embodiments mutually exclusive, unless so stated or as will bereadily apparent to those of ordinary skill in the art having thebenefit of the knowledge provided by this disclosure. Thus, the presentdisclosure includes a variety of combinations and/or integrations of theembodiments and examples described herein, as well as furtherembodiments and examples, as defined within the scope of all claimsbased on this disclosure, as well as all legal equivalents of suchclaims.

The accompanying drawings that form a part hereof, show by way ofillustration, and not of limitation, specific embodiments in which thesubject matter may be practiced. The embodiments illustrated aredescribed in sufficient detail to enable those skilled in the art topractice the teachings disclosed herein. Other embodiments may be usedand derived therefrom, such that structural and logical substitutionsand changes may be made without departing from the scope of thisdisclosure. This Detailed Description, therefore, is not to be taken ina limiting sense, and the scope of various embodiments is defined onlyby the appended claims, along with the full range of equivalents towhich such claims are entitled.

What is claimed is:
 1. An apparatus, comprising: a down hole tool housing attached to at least two ranging electromagnetic field strength sensors, each of the electromagnetic field strength sensors to measure electromagnetic field strength components associated with an electromagnetic field originating at a first well, via direct transmission or backscatter transmission, when the housing is disposed in a second well, to provide ranging measurements for calculating an approximate range between the first well and the second well; and a set of at least three reference electromagnetic field strength sensors attached in a circular path around the housing, to provide distorting field strength measurements.
 2. The apparatus of claim 1, wherein the ranging electromagnetic field strength sensors and the reference electromagnetic field strength sensors are spaced apart from each other in an azimuthal direction, and located at approximately a same radial distance from a longitudinal centerline of the housing.
 3. The apparatus of claim 1, wherein the ranging electromagnetic field strength sensors are spaced substantially equally apart from each other in an azimuthal direction.
 4. The apparatus of claim 1, wherein the reference electromagnetic field strength sensors are spaced substantially equally apart from each other along the circular path.
 5. The apparatus of claim 1, wherein the set of reference electromagnetic field strength sensors includes the two ranging electromagnetic field strength sensors.
 6. A system, comprising: a current source to couple current to a target well or a drilling well; and an apparatus comprising a down hole tool housing attached to two ranging electromagnetic field strength sensors, each of the electromagnetic field strength sensors to measure electromagnetic field strength components associated with an electromagnetic field originated by the current source, via direct transmission or backscatter transmission, when the housing is disposed in the drilling well, to provide ranging measurements for calculating an approximate range between the target well and the drilling well, and a set of at least three electromagnetic field strength sensors attached in a circular path around the housing, to provide distorting field strength measurements.
 7. The system of claim 6, further comprising: a range adjustment module to receive the ranging measurements and the distorting field strength measurements, and to provide adjusted values of the ranging measurements to calculate the approximate range.
 8. The system of claim 6, wherein the set of reference electromagnetic field strength sensors attached in the circular path around the housing includes the two ranging electromagnetic field strength sensors.
 9. An apparatus, comprising: a down hole tool housing attached to a ranging electromagnetic field strength sensor, the ranging electromagnetic field strength sensor to measure electromagnetic field strength components associated with an electromagnetic field originating at a first well, via direct transmission or backscatter transmission, when the housing is disposed in a second well, to provide ranging measurements for calculating an approximate range between the first well and the second well; and a reference electromagnetic field strength sensor providing a closed loop current path around the housing, to provide distorting field strength measurements.
 10. The apparatus of claim 9, wherein the reference electromagnetic field strength sensor comprises a toroidal antenna.
 11. The apparatus of claim 9, wherein the reference electromagnetic field strength sensor is attached to the ranging electromagnetic field strength sensor, or to the housing.
 12. A system, comprising: a current source to couple current to a target well or a drilling well; and an apparatus comprising a down hole tool housing attached to a ranging electromagnetic field strength sensor, the ranging electromagnetic field strength sensor to measure electromagnetic field strength components associated with an electromagnetic field originated by the current source, via direct transmission or backscatter transmission, when the housing is disposed in the drilling well, to provide ranging measurements for calculating an approximate range between the target well and the drilling well, and a reference electromagnetic field strength sensor attached in a closed loop path around the housing, to provide distorting field strength measurements.
 13. The system of claim 12, wherein the reference electromagnetic field strength sensor is used to measure the electromagnetic field strength components associated with the electromagnetic field originating at the first well, via the direct transmission or the backscatter transmission, in conjunction with the ranging electromagnetic field strength sensor.
 14. The system of claim 12, further comprising: a range determination module to receive the ranging measurements and the distorting field strength measurements, and to provide adjusted values of the ranging measurements to calculate the approximate range.
 15. A method, comprising: measuring electromagnetic field strength components associated with an electromagnetic field originating at a first well, via direct transmission or backscatter transmission, using two ranging electromagnetic field strength sensors attached to a housing, to provide ranging measurements; obtaining distorting field strength measurements using a set of at least three reference electromagnetic field strength sensors attached in a circular path around the housing; and determining the approximate range between the first well and a second well in which the ranging electromagnetic field strength sensors are disposed, based on the ranging measurements and the distorting field strength measurements.
 16. The method of claim 15, wherein the approximate range is determined according to the formula {right arrow over (H)}_(Target)={right arrow over (H)}_(Total)−{right arrow over (H)}_(Leakage), where {right arrow over (H)}_(Total) comprises a total field measurement at each sensor, wherein {right arrow over (H)}_(Leakage) comprises a field strength due to an insert current, and wherein {right arrow over (H)}_(Target) comprises a corrected field measurement.
 17. The method of claim 15, wherein the set of reference electromagnetic field strength sensors attached in the circular path around the housing includes the two ranging electromagnetic field strength sensors.
 18. A method, comprising: measuring a total electromagnetic field value for each one of a set of ranging sensors disposed in a second well, wherein a signal to be measured originates at a first well, and the measuring occurs in the second well; averaging the total electromagnetic field value over all of the ranging sensors to provide an average total field value; calculating an average distorting current value based on the average total field value; determining an average distorting field value based on the average distorting current value; determining a difference between the total electromagnetic field value measured by each of the ranging sensors and the average distorting field value, to provide corrected electromagnetic field value measurements; and determining an approximate range between the first well and the second well using the corrected electromagnetic field value measurements.
 19. A method, comprising: first measuring a total electromagnetic field value for a ranging sensor disposed in a second well, wherein a signal to be measured originates at a first well, and the first measuring occurs in the second well; second measuring a total electromagnetic field value of a reference sensor attached in a closed loop around a housing wherein the ranging sensor and the reference sensor are located on the same housing; calculating a distorting current value as a calculated distorting current value, based on the total electromagnetic field value of the reference sensor; determining a distorting field value for the ranging sensor based on the calculated distorting current value; determining a difference between the total electromagnetic field value measured by the ranging sensor and the distorting field value, to provide a corrected electromagnetic field value measurement; and determining an approximate range between the first well and the second well using the corrected electromagnetic field value measurement.
 20. A method, comprising: measuring electromagnetic field strength components associated with an electromagnetic field originating at a first well, via direct transmission or backscatter transmission, using at least one ranging electromagnetic field strength sensor attached to a housing, to provide ranging measurements; obtaining distorting field strength measurements using a reference electromagnetic field strength sensor attached in a closed loop path around the housing; and determining an approximate range between the first well and a second well in which the ranging electromagnetic field strength sensors are disposed, based on the ranging measurements and the distorting field strength measurements. 